1. Field of the Invention
The present invention relates to a discharge bulb that is used as a light source for an automobile headlamp, for example, and an arc tube that is used in the discharge bulb.
2. Description of Related Art
This kind of arc tube for a discharge bulb includes a pair of pinch seal portions, in each of which an electrode assembly having an electrode bar and a molybdenum foil joined in series is sealed, and a closed glass bulb that is a discharge light-emitting portion and in which a pair of electrodes are disposed to be opposed to each other and a luminous substance and the like are enclosed, the closed glass bulb being interposed between the pinch seal portions.
In the electrode assembly, the molybdenum foil and the electrode bar are joined by spot welding and examples of the spot welding include the resistance welding described in Japanese Patent Application Laid-Open (Kokai) No. 05-159744 (JP-A-05-159744) (see paragraphs 0009 to 0011, FIGS. 1, 2, and 5), for example.
In JP-A-5-159744, the overlapping portion of the molybdenum foil and the electrode bar is sandwiched by a pair of welding electrodes and a large electric current is caused to flow between the welding electrodes with the overlapping portion subjected to a high pressure exerted by the pair of welding electrodes, whereby welding is performed.
In particular, when the spot welding is performed, an angular end portion of the electrode bar and the molybdenum foil are pressed against each other, which tends to damage the molybdenum foil by causing a crack, for example, which in turn tends to cause breakage of the foil when the electrode assembly is pinch-sealed. Thus, the overlapping portion of the molybdenum foil and the electrode bar is spot welded at the point with a small margin left at the end of the electrode bar, thereby suppressing the damage of the molybdenum foil (breakage of the foil at the time of pinch sealing) caused by welding.
In JP-A-05-159744, however, a weld mark called a nugget can appear in the molybdenum foil in the form of a recess on the side opposite to the joint portion, at which the molybdenum foil and the electrode bar are joined with each other by spot welding and in this case, glass enters the nugget (recess) in the pinch seal portion. When thermal stress concentrates at the entering portion, at which the glass layer enters the nugget, at the time of turning on or off the bulb (arc tube), a crack can occur in the vicinity of the nugget, which can lead to reduction in lifetime of the bulb.
In particular, in the case of the mercury-free arc tube, in which mercury effective in raising the tube voltage is not enclosed in the closed glass bulb, in order to compensate for the reduced tube voltage as compared to the mercury-containing arc tube, the tube current is increased to obtain enough power and the large electric current imposes a high load on the electrode (bar). When such a large electric current makes the temperature of the electrode (bar) high, the thermal stress (thermal strain) that occurs at the interface between the molybdenum foil and the glass is large and the crack that runs from the glass portion near the nugget occurs more easily.